Structure of ice IV, a metastable high-pressure phase

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Abstract
Ice IV, made metastably at pressures of about 4 to 5.5 kb, has a structure based on a rhombohedral unit cell of dimensions aR = 760±1 pm, α = 70.1±0.2°, space group R3̄c, as observed by x‐ray diffraction at 1 atm, 110 K. The cell contains 12 water molecules of type 1, in general position, plus 4 of type 2, with O(2) in a special position on the threefold axis. The calculated density at 1 atm, 110 K is 1.272±0.005 g cm−3. Every molecule is linked by asymmetric H bonds to four others, the bonds forming a new type of tetrahedrally‐connected network. Molecules of type 1 are linked by O(1)⋅⋅⋅O(1′) bonds into puckered six‐rings of 3̄ symmetry, through the center of each of which passes an O(2)⋅⋅⋅O(2′) bond between a pair of type‐2 molecules, along the threefold axis. The six‐rings are linked laterally by type‐2 molecules to form puckered sheets that are topologically similar to such sheets in ice I, but are connected to one another in a very different and novel way. One quarter of the intersheet bonds connect not directly between adjacent sheets but remotely, from one sheet to the second nearest sheet, through holes in the intervening sheet. These remote connections are the O(2)⋅⋅⋅O(2′) bonds, passing through the O(1)‐type six‐rings. The sheets are stacked in a sequence based on ice Ic, modified by reversal of the puckering to form the remote connections and by internal distortion of the sheets to complete the remaining intersheet bonds. Of the four nonequivalent H bonded O⋅⋅⋅O distance in the structure, two (279 and 281±1pm) are only moderately lengthened relative to the bonds in ice I (275 pm), whereas the O(1)⋅⋅⋅O(1′) bond (288±1pm) and O(2)⋅⋅⋅O(2′) bond (292±1pm) are lengthened extraordinarily. This is caused by repulsion between O(1) and O(2) at nonbonded distances of 314 and 329 pm in the molecular cluster consisting of the O(1)‐type six‐ring threaded by the O(2)⋅⋅⋅O(2′) bond. The mean O⋅⋅⋅O bond distance of 283.3 pm, which is high relative to other ice structures except ice VII/VIII, reflects similarly the accommodation of a relatively large number (3.75 on average) of nonbonded neighbors around each molecule at relatively short distances of 310–330 pm. Bond bending in ice IV, as measured by deviation of the O⋅⋅⋅O⋅⋅⋅O bond angles from 109.5°, is relatively low compared to most other dense ice structures. All H bonds in ice IV except O(1)⋅⋅⋅O(1′) are required to be proton‐disordered by constraints of space‐group symmetry. The x‐ray structure‐factor data indicate that O(1)⋅⋅⋅O(1′) is probably also proton‐disordered. Ice IV is the only ice phase other than ice I and Ic to remain proton‐disordered on quenching to 77 K. The increased internal energy of ice IV relative to ice V, amounting to about 0.23 kJ mole−1, which underlies the metastability of ice IV in relation to ice V, can be explained structurally as a result of extra overlap and bond‐stretching energy in ice IV, partially compensated by extra bond‐bending energy in ice V. The structural relation between ice IV and ice I offers a possible explanation for the reduced barrier to nucleation of ice IV, as compared to ice V, in crystallizing from liquid water.

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